Many propulsion systems, such as gas turbine engines, are based on the Brayton Cycle, where air is compressed adiabatically, heat is added at constant pressure, the resulting hot gas is expanded in a turbine, and heat is rejected at constant pressure. The energy above that required to drive the compression system is then available for propulsion or other work. Such propulsion systems generally rely upon deflagrative combustion to burn a fuel/air mixture and produce combustion gas products which travel at relatively slow rates and constant pressure within a combustion chamber. While engines based on the Brayton Cycle have reached a high level of thermodynamic efficiency by steady improvements in component efficiencies and increases in pressure ratio and peak temperature, further improvements are welcomed nonetheless.
Accordingly, improvements in engine efficiency have been sought by modifying the engine architecture such that the combustion occurs as a detonation in either a continuous or pulsed mode. The pulsed mode design involves one or more detonation tubes, whereas the continuous mode is based on a geometry, typically an annulus, within which single or multiple detonation waves spin. For both types of modes, high energy ignition detonates a fuel/air mixture that transitions into a detonation wave (i.e., a fast moving shock wave closely coupled to the reaction zone). The detonation wave travels in a Mach number range greater than the speed of sound (e.g., Mach 4 to 8) with respect to the speed of sound of the reactants. The products of combustion follow the detonation wave at the speed of sound relative to the detonation wave and at significantly elevated pressure. Such combustion products may then exit through a nozzle to produce thrust or rotate a turbine. With various rotating detonation systems, the task of preventing backflow into the lower pressure regions upstream of the rotating detonation has been addressed by providing a steep pressure drop into the combustion chamber. However, such may reduce the efficiency benefits of the rotating detonation combustion system.
Generally, a detonation combustion system is based on whether a minimum quantity of detonation cells can be sustained in an annular combustion chamber. The detonation cell is characterized by a cell width (λ) that depends on the type of fuel and oxidizer as well as the pressure and temperature of the reactants at the combustion chamber and the stoichiometry (4)) of the reactants. For each combination of fuel and oxidizer, cell size decreases with increasing pressure and temperature, and for stoichiometry greater than or less than 1.0. In various propulsion system apparatuses, such as for gas turbine engines, the cell width may decrease by 20 times or more from a lowest steady state operating condition (e.g., ground idle) to a highest steady state operating condition (e.g., maximum takeoff).
It is generally known in the art that combustion chamber geometry is defined by a desired detonation cell size based on the fuel-oxidizer mixture and the pressure, temperature, and stoichiometric ratio thereof. Various combinations of fuel-oxidizer mixture, pressure, temperature, and stoichiometric ratio (e.g., at various operating conditions of the propulsion system) may render a fixed geometry combustion chamber inefficient at more than one operating condition. However, variable geometry combustion chambers generally involve complex structures that may significantly reduce or eliminate overall propulsion system efficiency or operability.
Rotating detonation combustors are generally annular and require fuel-oxidizer injection to approach pre-mixed conditions in minimal length while mitigating flameholding within the combustion combustor. Thus, rotating detonation combustion systems generally require a plurality of minimally sized orifices for rapid mixing of fuel and oxidizer within the injector. The liquid fuel also needs to be atomized into very small droplets for the rotating detonation wave to be effected. Liquid fuel atomization may generally be due to momentum transfer from the air streams of the injector.
However, such known geometries generally inhibit application of rotating detonation combustion systems into increasingly smaller apparatuses, or generally require tight-tolerance features that increase the complexity of manufacturing of the combustion system, each of which may limit application of rotating detonation combustion systems.
Therefore, there is a need for a detonation combustion system that provides improved liquid fuel atomization. Additionally, there is a need for a detonation combustion system that provides a desired detonation cell size across a plurality of operating conditions.